The type, shape, size and dimensional tolerance of the part to be inspected determine the kind of equipment to be considered. Flat parts (sheet metal stampings, plastic, rubber, etc.) can be inspected faster with a video system. An optical comparator is likewise a 2-D device, best for checking profiles or small flat parts by hand. They are not suited for production.

Height Gages, as the name implies, are good for checking heights on a surface plate. Inspecting a hole pattern with this 1-D device by flipping the part, still practiced by some, is cumbersome, time consuming and unreliable. In this day and age, when a small CMM does not cost more than some height gages, this is a truly wasteful approach.

For 3-D measurements the CMM should be the equipment of choice. A motorized CMM with a powerful computer and software does cost just a few thousand dollars more today than did a manual CMM of the same size twenty five years ago. They were equipped with a digital readout only. The variety and configuration of sensors available today give the user the possibility to inspect just about any type of part, be it a complex aircraft valve body, a plastic or rubber part, a glass lens or polished mold. Line lasers, capable of taking thousands of points per second can digitize an odd shaped part in minutes.

In short, CMMs cover the whole gamut from simple to use manual units with a basic touch probe and software to fully automatic machines in a production environment pre-programmed to inspect the most sophisticated of parts to a fair accuracy. You can have anything in between. Like the survival of the fittest, time has removed inferior designs from the market.

Of the many styles out there the traveling bridge, the gantry and the horizontal arm design are most common with the traveling bridge being the most popular. Horizontal arm units, capable of measuring large envelopes at acceptable accuracies are used mainly for large auto body parts, large weldments, etc.

When selecting a CMM the following issues should be addressed first before looking at accessories: a) size, b) manual or DCC (CNC), c) Inspection room or shop floor application.

a) The size is obviously determined by your largest part.

Then again if this particular part shows up only 2 times a year, you should reconsider. The smaller the unit the better. On the other hand you also have to look at the configuration of the parts. If you have a part that is 12” wide, for instance, with 3” deep bores on either side that you wish to access with an articulating head, you need an additional 5” of travel on each side. This brings the total travel to 22” with no safety clearance added. Pre-qualifying these positions on the reference sphere further adds to the necessary envelope.

b) Manual or DCC.

Financial issues aside, this is largely a function of part quantity or a combination of quantity and complexity. A prototype shop should have a manual unit and a production environment requires a DCC unit. Aside from the fact that CNC units have dramatically come down in price, medium part quantities e.g. 10-30 can be efficiently checked manually with a pre-written program. An unskilled operator then simply touches the points on the part as commanded by the screen. The in and out of tolerance condition may be printed out.

A ROI calculations sheet is available for cross-over quantities making a decision as to manual or DCC easy.

If a part is complex and small, with fine features, a DCC unit is recommended even for small quantities. It is hard to negotiate a .5mm stylus into a 1.5mm hole without breaking it.

c) Shop floor or inspection room application.

Since an inspection room has to cover all eventualities it should be equipped with a fair size unit, the top of the line software offered by the OEM, an indexable probe head (manual or motorized) and a good selection of styli and extensions. This requires a well trained and/or experienced operator. Any new operator should be well trained by the OEM.

When selecting a shop floor CMM robustness of the equipment has to be considered. Mechanical bearing units are inherently more reliable, since they do not require a constant supply of dry and clean air which can create a maintenance headache. Air bearing CMMs are a poor choice for applications in a dirty environment, requiring an expensive enclosure that a mechanical bearing unit with covers and bellows can do without. The potential user should educate himself on this issue.

Production CMMs are usually dedicated to one or just a few specific parts. Parts should be fixtured. Fixtures can be supplied by the user or the OEM. The same goes for the part programs. Probing systems should be as simple as possible. Motorized probe heads should be avoided in high production situations if the part is not too complex. Probes with detachable stylus modules and a stylus rack may accomplish the same task at half the price if no more than 6-8 orientations are required. Angled styli orientations can be created with stylus knuckles.

Fast start menus make the operation and program selection easy for shop floor personnel.

If the temperature in the shop varies substantially from 68°F (20°C), the standard OEM calibration temperature, you may want to look at a temperature compensation package or purchase a unit with metal scales on metal structures if you inspect metal parts. It is always a good practice to let the part soak to reach the same temperature as the CMM.

Sensors

The electronic touch trigger probe, the scanning probe, the single point laser, the line laser and the video camera are the sensors offered on CMMs today.

Touch Trigger Probes, being the least expensive yet capable of measuring just about everything on a machined part, are used in the majority of applications. They usually consist of a probe head, fixed or indexable, the touch probe itself and the styli. DCC machines may be outfitted with a motorized indexable head (7 ½°), but add substantially to the overall cost. For a few thousand dollars more you can purchase a small DCC CMM.

Probes with detachable stylus modules are a good investment, especially for DCC units. They allow the use of a stylus rack (6 stalls) akin to a tool changer on a machining center. Modules are held in place magnetically and detach in the event of a collision without damaging the probe itself. A multitude of styli (measuring tips) are available from several sources that cover just about any measuring task, from a .3mm ball tip to a 1 diameter disc to a cylinder for thin sheet metal.

Mechanical Scanning Probes are used to gather a high number of points in bores and surfaces of prismatic parts and for digitizing non-linear unknown surfaces. Higher density points give you a more accurate picture of the feature as required by ANSI 14.5 e.g. roundness, cylindricity and flatness. These probes are obviously more expensive than trigger probes and require a high end controller.

The Single Point Laser is also used for digitizing. It is an excellent tool to check the profile of delicate surfaces e.g. coated optics and soft parts since it does not physically touch the part.

The Line Laser is the fastest way to digitize or inspect non-linear surfaces and contours like cell phone housings or car body parts. The lines are up to 2 wide taking 4000 or more points per second. The accuracies range from + .001 to + .00025. It is a powerful tool in conjunction with CAD software having a 3-D best fit option. The line laser is popular for reverse engineering.

Video attachments for CMMs require additional software and back-lighting and are therefore not widely used.

Software

Measuring software is the most important part of a CMM next to the physical structure. It may be your main purchasing criteria. Most OEM’s have their own brand, some do not. You should inquire as to how long the software has been on the market. It is very hard to evaluate CMM software in just a couple of hours unless you are an experienced CMM operator. The big names in the industry do not necessarily have the easiest to use systems, which is what the buyer should be looking for. Whichever software takes the least number of keystrokes or mouse clicks to measure a feature or a complete part is the better one. Fancy graphics and many windows do not measure a part. If time permits take a somewhat complex part to the vendors and compare inspection and DCC programming time as well as the ease to do so.

Beyond user friendliness you may need other features like real time SPC, export to CAD or a 4th axis.

You should be cautious with the much talked about CMM program writing from CAD. Its O.K. for simple parts, but a DCC motion program with a motorized head and widely varying styli is fraught with pitfalls. If and when it has progressed to a point where its close to being seamless, it would require quite a knowledgeable operator. Most people do not recognize the fact that a CMM is not a simple single point system like a machine tool with only one fixed and defined coordinate system where the part to be machined is always aligned to the axis travels. Therefore a CMM programmer, not necessarily the operator has to have a good grasp of 3-D points in space and coordinate system transformations. Good and thorough training of the programmer is imperative. Once a program is written a lesser trained person may push the buttons. There is many a CMM that sits in a corner because nobody knows how to use it.

When evaluating software you also may inquire about the platform (Windows and C++) and whether there are service contracts with future upgrades available. Proper support is important.

Accuracy

The potential user should understand the difference between resolution, repeatability, and accuracy. In brief; resolution is the least count of the measuring system. Repeatability is how well the CMM repeats a given dimension or feature; this is always some multiple of the resolution and includes the non-repeatability of the probe, which in some instances exceeds that of the CMM itself. Linear accuracy, taken along each axis travel, is how much any linear dimension deviates from the absolute NIST standard. The volumetric accuracy is usually determined with a ball bar according to B89.4.1a or another artifact and includes the non-linearity of the ways, out of squareness condition, length variation of all axes to one another as well as the non-repeatability of the probe. Consequently this number is substantially higher than the linear accuracy.

Some CMMs are highly software compensated to achieve the stated accuracy. There is nothing wrong with taking out the last wrinkles of an otherwise sound structure, but making computer compensation more or less the basis of the CMM accuracy is not commendable. If you have a collision, lose the compensation table or upgrade to a better software system down the line, re-mapping the CMM will be expensive. CMMs with intrinsic accuracy have the lowest maintenance cost over their life span.